The present invention relates to switching of packets of data, and in particular to the switching of such data packets by means of a packet switch between digital subscriber or trunk lines.
Normally an interface between a group of subscriber lines and a trunk line comprises a packet switch which includes a plurality of subscriber line cards. Each card accepts data from a number of subscriber lines, usually in the form of X.25 packets, and may convert the data into smaller packets with appropriate protocols and codes for routing around the packet switch. It is often necessary to route the packets thus formed to another card for transmittal to an output port. Usually a single data bus is provided (for example, the "Multibus" or "VME") interconnecting the line interface cards, but this raises the problem of unreliability, since if one card fails, then the data bus will also fail. One solution to this is the use of a second bus in parallel with the first bus, but although this reduces the problem of bus failure, it does not eliminate the problem. A further problem is that of growth of the system, since as the number of subscribers and hence line interface cards grows, the traffic across the bus may increase to exceed the bus capacity.